(1) Field of the Invention
The present invention belongs to the technical field of image forming apparatus and more detailedly relates to an image forming apparatus such as a copier, printer, etc., which uses a transfer roller for transferring the toner image from an image support to print media.
(2) Description of the Prior Art
Conventionally, in image forming apparatus such as copiers, printers, etc. which use the so-called method of electrophotography using a toner as the developer, the corona transfer method, which effects corona discharge from the rear surface of the print medium, has been widely used in order to transfer the toner image formed on the photosensitive member to the print medium such as recording paper etc.
This corona transfer method needs application of a high voltage of some kilo volts in order to cause corona discharge. Therefore, this configuration needs a high voltage circuit and insulating countermeasures, resulting in high cost for the apparatus. Further, there is a problem in that ozone which is generated from electric discharge oxidizes and damages the apparatus components, especially causing shortening the photosensitive member's life.
In order to solve this problem, a roller transfer method has been proposed in which a cylindrical transfer roller is closely abutted against the photosensitive member with the recording paper in between. This roller transfer method performs transfer of the toner image by bringing the conductive transfer roller set at a fixed voltage into the rear surface of the recording paper. Typically, a voltage of some hundred volts to about 2.0 KV is applied to the transfer roller so as to perform the transfer operation.
Since the roller transfer method will generate no or less ozone compared to the corona transfer method, this method is beneficial for environmental preservation. Further, the voltage applied to the transfer roller can be set at voltage lower than that of the conventional configuration, a high voltage board for high voltage application can be made compact. Moreover, there is another advantage of less toner scatter and less disturbance of the image since the recording paper can be in close contact with the photosensitive member.
With the development of information processing devices towards personal use, image forming apparatus have been made simple, compact and low-priced but still there have been a strong demand for an image forming apparatus such as an electrophotographic copier, printer, and the like which is able to perform a stable transfer operation using a low voltage. For these reasons, many of recent, compact printers use the roller transfer method as stated above to make the apparatus compact.
Up to now, various types of compact printers using transfer roller configurations have been proposed. For example, Japanese Patent Application Laid-Open Hei 2 No.173677 discloses an image forming apparatus in which the length of the charging device is set greater than the length of the transfer device while the maximum print media width is set smaller than the length of the transfer device in order to protect the edges of the charging device from dirt and enable uniform charging over the image support surface over a prolonged period. FIG. 1 shows a case where the maximum width W1 of the print medium P (print paper) is set smaller than the transfer roller length W2.
However, the conventional roller transfer techniques have the problem in that the combined effect of the abutment force for pressing the transfer roller against the photosensitive member and hard substances such as calcium carbonate etc., contained in the print media lessens the coating thickness of the photosensitive member with the augmentation of the image forming in number, leading to lowering of the electrified potential and lowering of the surface potential after exposure hence causing marked degradation of the image quality.
FIG. 2 is a chart showing the relationship between the operating time of a photosensitive member and its coating thickness. For example, it is understood that when the operating time of the photosensitive member was 250 K seconds, the coating thickness of the photosensitive member was 20 .mu.m (point P in the chart) and the coating thickness reduced to 15 .mu.m at the operating time of 420 K seconds (point Q in the chart).
In this case, as the operating time of the photosensitive member (the number of image formations) increased, the coating thickness which had been 25 .mu.m at the initial stage reduced to 5 .mu.m or lower after 600 K seconds. FIG. 3 is a chart schematically showing the relationship between the surface potential and residual potential over the above period. As understood from this chart, with increase in the operating time of the photosensitive member, the surface potential reduced stepwise while the residual potential increased stepwise resulting in decrease in the difference between the surface potential and the residual potential.
In an image forming apparatus of a reversal development type, which is currently predominant, as shown in FIG. 4, as the electrified potential V0 of the photosensitive member decreases with the reduction in coating thickness of the photosensitive member, the difference between the electrified potential V0 and the developing bias DVB, or so-called background margin decreases, causing fogging in the non-image area and degradation of the image quality.
If fogging occurs as above, extra toner which is not needed for development will adhere to the photosensitive member surface, which increases toner consumption and hence the running cost and servicing cost. Further, this will also degrade and abrade the cleaning blade at an early stage.
Further, as shown in FIG. 5, the transfer roller is urged at its ends, upward in the figure with urging members such as springs etc., so as to abut itself against the photosensitive member. Both ends of the transfer roller come in close contact with the photosensitive member because of their being close to the points urged by the springs while the mid part of the transfer roller is set warped so as to be away from the photosensitive remember.
Particularly, since the contact pressure at the both ends of the photosensitive member is greater than that at the mid part thereof, hard substances such as calcium carbonate etc., are liable to come off from both edges of the print media and adhere to the photosensitive member, so that combination of the mechanical pressure and the abrasive functions of the hard particles accelerates the wear at both ends of the photosensitive member, causing difficulties in maintaining stable image quality over a prolonged period of time.